After much research, I have a lot of conflicting advise about the need to add a vapor barrier before installing the backer (hardy / cement) board for my bathroom shower.

The "expert" at Home Depot, and another guy on youtube seem to think this is a very good idea, and even suggest sealing it with roofing tape. (The guy on Youtube tapes it directly to the sill of tub.)

However, when I asked about the tape and how far down I should go at Lowes, they told me they'd never hear of anyone doing that, and that it didn't seem like a good idea. Likewise, Ask This Old House only recommends putting down caulking on the sill before applying the backer board, and then again after before adding tile, and then caulking a 3rd time under the tiles before grouting.

Should I add a vapor barrier between the frame and the cement board? Why or why not?

7 Answers
7

The current best practice is to place waterproofing OVER the backetboard and just UNDER the tile. Thin sheet membranes like Kerdi or NobleFlex are examples
. Paint on membranes can also be used, such as RedGuard or AquaDefense.

This a is a recent job I completed illustrating Kerdi over Hardibacker.

While I agree the waterproofing should be on the inside as much as possible (I've used Redgard and like it) the use of plastic sheeting behind the cement board has been common practice (at least in the US) for a long time and may even be code in places (if you're not using the alternatives you've listed)
– DA01Mar 4 '14 at 3:25

@DA01 is it also standard practice to leave gaps in the caulking to let the water drain from?
– virtualxtcMar 4 '14 at 8:03

3

@virtualxtc it depends on the system you are installing, but most modern tubs have a lip that the plastic and cement board would overlap. Whether or not to caulk that spot is debatable. Some say that you want any water behind to eventually drain. Other's say that water should never get there to begin with and the plastic is just a last line of defense. (FWIW, I push Redgard as a better alternative, as the waterproofing is now closer to the water)
– DA01Mar 4 '14 at 15:28

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Do I put a plastic sheet up so local inspector doesn't throw a fit? Yes. Is it totally useless with HB? Yes. HB is basically waterproof. I have let these boards sit in water for months with no affect so why do I need a sheet behind it? (Little secret of mine... I might accidentally puncture said plastic in several places while installing HB on top) - The whole point is where is the water coming from that the plastic would help? I am more worried about the wall on the other side of plastic sheet - if water is trapped in plastic sheet for a long time, mold could grow on nearby walls.
– DMooreMar 4 '14 at 16:56

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If he uses a product like durock, he must apply a barrier to protect the studs. This could be something like kerdi (in your example) or redgard or a traditional barrier but never more than one of these.
– MatthewNov 24 '14 at 22:25

I called USG,the maker of Durock cement board Next Gen, and they advised using no vapor barrier so that the wall cavity can breath and allow any moisture to evaporate. Their online instructions/diagrams show no use of a vapor barrier.

You should, so that any moisture that gets behind the cement board runs into the tub or shower instead of into your wall cavity (causing rot or mold/mildew).

If you overlap the moisture barrier with the raised lip around the tub, any moisture has only one path to take due to gravity -- down the barrier and into the tub. I usually use thick-mil plastic sheeting, overlapped in shiplap fashion (overlapping, laid down from bottom to top -- like shingles).

Do caulk between the tub and tile, but leave small gaps for any water runoff down the sheeting to escape. I always take a level to the tub base to see where the water would run to if it had the chance, and place my holes appropriately.

You'll staple the sheeting to the framing -- don't worry about these minor penetrations of the barrier.

I'm confused about the shiplap, can one can buy rabbited thick-mil plastic sheeting? and I assume you mean horizontal not vertical shiplap such that the water will run down, correct? If I caulk the tub, wouldn't that just make it puddle behind the cement board? What tape should I use? water-tight aluminum? tyvek tape?
– virtualxtcMar 3 '14 at 13:59

The shiplap reference simply refers to the upper layer covering the edge of the lower layer, like roof shingles. Gravity does the water proofing. Although I am not an advocate of solid impermeable membranes, the small area behind the tile, should not create any mildew problems with the blockage of moisture that will need to pass through the walls. I just renovated my 1989 hall bath shower, tile over Wonderboard. When I pulled it down to the studs. there was no evidence at all of any water passing through the 4X4 tile. except in one corner where I never nailed the corner tight. I fixed that.
– JackMar 3 '14 at 15:27

When you caulk around the tub, you need to leave small 1cm gaps every so often in the caulk for water to escape.
– EtherealMar 3 '14 at 21:06

Ask TOH is usually right on, but how old was the segment? A vapor barrier behind the hardi-backer or concrete board is SOP today. These walls are also insulated for sound proofing so even more important to water seal the outer layer from the inner cavity. If the plastic (6 mil min) is not large enough to cover the entire envelope in one piece, it should be installed at the bottom first, overlapped so water runs over the lap downhill, and taped at all joints.
– shirlock homesMar 3 '14 at 23:47

@Ethereal: Your comment about leaving 1cm gaps is relevant to me. I built a shower last year and wondered how any water coming down the vapor barrier would get into the shower base because I caulked the gap where the vapor barrier lipped over the shower base's outside edge. Having read your comment I'm now wondering whether I should drill some small weep holes through the caulk. Would you?
– getterdunMar 4 '14 at 2:16

plastic behind cement board with waterproofing is essentially suspenders when you already have a belt. the waterproofing is also a vapor retarder so if the red guard or kerdi product is continuous, lapping corners and all, the plastic will be unnecessary.

if you think of tile like exterior cladding, it would make sense to leave an open hole in the sealant in line with a vertical joint in the grout. porcelain and ceramic tile is impervious (some soft specialty limestone or sandstones are not), so any water that migrates does so in the grout joints (which crack on occasion as you know). it should then hit the waterproofing and migrate down (however slowly) to the "weep" in the tile joint around the lip of the tub/shower. ideally, you would install the vertical tile over vertically troweled mortar, which effectively provides little paths for the water to drain within the wall. EIFS manufacturers have tried and tested this strategy such that it is their only warranted water management technique.

this is why you eventually see black mildew stains in the grout around the base of the tile - because the water is stuck back there. the redguard product is a vapor retarder which means the steam (water vapor) from the shower won't migrate thru the substrate, it will hit the waterproofing and, assuming it condenses on the colder surface migrate down along the same path noted above. in most instances, the tile is colder than the cement board, so the vapor condenses on it first and stays on the exposed surface. rather than continuous plastic sheet.

as compromise, I would suggest a 6" high strip stapled above the flange and lapped over the flange and under the edge of the cement board is an adequate solution to prevent overflow situations. probably unnecessary on a tub, but possibly on a shower pan. then waterproof the walls, install the mortar with vertical troweled grooves, set the tile, wait, install sealant under each tile, stopping at each vertical joint for a small gap, plug the gap with a spacer before grouting, then fully grout, remove the spacers, and clean the tile. the holes will look a bit odd at first, but will serve their purpose.

this comes from me, an architect, who spends a lot of time thinking about how to keep water out of a building. I was told once "think like water" and you'll figure out where to let it out.

no flange provides a different challenge. i'm about to rebuild my own shower with a tile floor and am thinking thru these exact details before buying anything. thanks for talking about it and making me think like water for an interior application.

This is a much-delayed answer, but in case it might be useful to others, the first consideration needs to be the manufacturer's own testing and resulting instructions. Most of the answers above contradict what the cement board manufacturers themselves recommend, so if you use them, you're on your own. This includes applying various sealants between the board and the tile which Hardiebacker, for example, does not recommend for interior installations.

If you are working in new construction and have a building engineer involved in the design or construction phase of the project, it may be worth asking whether the wall design will be primarily drying to the interior or exterior. For example, if the cavity is going to be filled with sprayed-in, closed-cell foam, any moisture that gets into the cavity may have nowhere to go and the engineer may recommend an alteration to the manufacturer's instructions for typical installation in order to suit the particular circumstance.

Sealants do break down over time and if you're worried about bulk water eventually making it back through the joints in the board (where you do want to be leaving small gaps to allow for movement in the wall without cracking or dislodging your finish work), you can always use asphalt roofing felt behind the board, overlapped as on the roof, which can be used to redirect the water to the sill but which is not a very effective vapor barrier.

Also, make sure you're using the right thickness of board for the application. I have seen folks try to use nom. 1/4" board where nom. 1/2" board was specified by the manufacturer for the application.

It does retain moisture, but the advice given by others here is to move the waterproofing between the tile and cement board with products like Redguard or Ditra. If there's a waterproof barrier there, you do not want a second vapor barrier in the wall, that would create a trap for the moisture. Also, the linked advice appears to be for drain pans, where you can get away with less waterproofing behind the tile.
– BMitch♦Sep 10 '15 at 21:37